1. Field of the Invention
Generally, the present invention relates to the formation of integrated circuits, and, more particularly, to the formation of metallization layers including highly conductive metals, such as copper, embedded into a dielectric material including a capping layer acting as an efficient diffusion barrier layer for the metal.
2. Description of the Related Art
In modern integrated circuits, minimum feature sizes, such as the channel length of field effect transistors, have reached the deep sub-micron range, thereby steadily increasing performance of these circuits in terms of speed and/or power consumption. As the size of the individual circuit elements is significantly reduced, thereby improving, for example, the switching speed of the transistor elements, the available floor space for interconnect lines electrically connecting the individual circuit elements is also decreased. Consequently, the dimensions of these interconnect lines have to be reduced to compensate for a reduced amount of available floor space and for an increased number of circuit elements provided per chip.
In integrated circuits having minimum dimensions of approximately 0.35 μm and less, a limiting factor of device performance is the signal propagation delay caused by the switching speed of the transistor elements. As the channel length of these transistor elements has now reached 0.1 μm and significantly less, it turns out, however, that the signal propagation delay is no longer limited by the field effect transistors, but is limited, owing to the increased circuit density, by the interconnect lines, since the line-to-line capacitance is increased and the conductivity of the lines is reduced due to their reduced cross-sectional area. The parasitic RC time constants, therefore, require the introduction of new types of material for forming the metallization layer.
Traditionally, metallization layers, i.e., the wiring layers including metal lines and vias for providing the electrical connection of the circuit elements according to a specified circuit layout, are formed by a dielectric layer stack including, for example, silicon dioxide and/or silicon nitride with aluminum as the typical metal. Since aluminum exhibits significant electromigration at higher current densities than may be necessary in integrated circuits having extremely scaled feature sizes, aluminum is being replaced by copper, which has a significantly lower electrical resistance and a higher resistivity against electromigration. For highly sophisticated applications, in addition to using copper and/or copper alloys, the well-established and well-known dielectric materials silicon dioxide (k≈4.2) and silicon nitride (k>5) may be replaced by so-called low-k dielectric materials. However, the transition from the well-known and well-established aluminum/silicon dioxide metallization layer to a copper-based metallization layer, possibly in combination with a low-k dielectric material, is associated with a plurality of issues to be dealt with.
For example, copper may not be deposited in relatively high amounts in an efficient manner by well-established deposition methods, such as chemical and physical vapor deposition. Moreover, copper may not be efficiently patterned by well-established anisotropic etch processes. Therefore, the so-called damascene or inlaid technique is frequently employed in forming metallization layers including copper lines and vias. Typically, in the damascene technique, the dielectric layer is deposited and then patterned for receiving trenches and via openings that are subsequently filled with copper by plating methods, such as electroplating or electroless plating. This damascene technique for forming copper-based metallization layers in standard dielectric materials, such as silicon dioxide, and a plurality of low-k dielectrics, typically requires the formation of a diffusion barrier layer at interfaces with the neighboring dielectric material as copper readily diffuses in a plurality of dielectrics, such as silicon dioxide, and in many low-k dielectrics. Moreover, the diffusion of moisture and oxygen into the copper-based metal has to be suppressed as copper readily reacts to form oxidized portions, thereby possibly deteriorating the characteristics of the copper-based metal line with respect to adhesion, conductivity and the resistance against electromigration. For instance, silicon nitride is known as an effective copper diffusion barrier and may be used, for instance, as a capping layer. In other cases, when the moderately high permittivity of silicon nitride is considered inappropriate, nitrogen enriched silicon carbide (SiCN) is frequently used as a copper diffusion barrier. Despite the diffusion hindering effect of the silicon nitride capping layers and silicon carbide-based capping layers, it turns out, however, that copper's resistance against electric current induced material transport (electromigration) strongly depends on the characteristics of an interface between the copper-based metal and the adjacent capping layer. Therefore, in sophisticated integrated circuits featuring high current densities, it is generally preferable to design the deposition process for the capping layer such that a desired high adhesion and thus high performance with respect to electromigration is achieved. For this purpose, corresponding deposition techniques with preceding plasma-based cleaning steps are typically used. With reference to FIGS. 1a and 1b, a typical conventional process flow for forming a SiCN capping layer will now be described in more detail.
FIG. 1a schematically shows a cross-sectional view of a semiconductor device 100 prior to the formation of a silicon nitride or silicon carbide nitride based capping layer on a copper-containing metal region. The semiconductor device 100 comprises a substrate 101 that may include circuit elements, such as transistors, capacitors and the like, which for convenience are not shown. Moreover, the substrate may have formed thereon one or more metallization layers, i.e., dielectric layers in which metal-filled lines and vias are embedded to establish the required electrical connections between the individual circuit elements. For convenience, a single copper-containing metal region 103 is illustrated to represent any copper-based metal lines of one or more metallization layers. The copper-based metal region 103 may be embedded in any appropriate dielectric material, such as silicon dioxide, fluorine-doped silicon dioxide, a low-k material such as hydrogen enriched silicon oxycarbide (SiCOH), appropriate polymer materials or any combination thereof. As previously explained, copper may readily diffuse in a plurality of dielectric materials and hence a conductive barrier layer 102 is typically provided between the dielectric material of the substrate 101 and the copper-containing material of the region 103. The barrier layer 102 may be comprised of two or more individual layers to provide the required characteristics not only in view of the copper diffusion blocking effect, but also with respect to adhesion to the surrounding material, and the diffusion blocking of oxygen, fluorine and other reactive species in to the copper region 103. For example, tantalum, tantalum nitride, titanium, titanium nitride and combinations thereof are frequently used for appropriate materials for the barrier layer 102.
A typical process flow for forming the semiconductor device 100 as shown in FIG. 1a may comprise the following processes. After the formation of any circuit elements, which may exhibit extremely scaled critical dimensions in sophisticated applications, such as a gate length of field effect transistors in the range of approximately 50-100 nm and even less, one or more metallization layers may be formed in accordance with well-established single damascene or dual damascene techniques. That is, an appropriate dielectric layer stack including an etch stop/barrier layer similar to the capping layer that is to be formed on top of the copper-based metal region 103 may be deposited, followed by another dielectric layer, such as silicon dioxide or a low-k dielectric material, which may be formed by well-established techniques, such as plasma enhanced chemical vapor deposition (PECVD), spin-on techniques and the like.
Thereafter, the dielectric layer stack may be patterned by photolithography and etch techniques to form trenches and vias in the dielectric layer stack, wherein the lower etch stop/barrier or capping layer (not shown) may be used as an etch stop. For convenience, only a single trench may be considered for the further explanation, in which the copper-based metal region 103 is to be formed. Thus, after the patterning of the corresponding trench, the barrier layer 102 may be deposited on the basis of well-established deposition techniques, such as sputter deposition, chemical vapor deposition (CVD), atomic layer deposition (ALD) and the like. For instance, well-established recipes for the sputter deposition of tantalum and tantalum nitride as well as titanium and titanium nitride may be used to form the barrier layer 102 having the desired characteristics.
Thereafter, a seed layer (not shown) comprised of, for instance, copper may be formed by sputter deposition or any other appropriate deposition technique. Based on the seed layer, copper may be deposited by, for instance, electroplating or electroless plating, thereby reliably filling the previously formed trenches and vias and also forming the copper-based metal region 103.
Next, excess material deposited during the preceding electrochemical deposition process, as well as the seed layer and the conductive barrier layer 102 formed on portions outside the copper-based metal region 103, may be removed to provide the electrically insulated copper-based metal region 103. For this purpose, a removal process typically comprising a chemical mechanical polishing (CMP) process may be performed, during which a surface 103A of the region 103 is exposed, which may thus be subjected to any chemical reactions, resulting in discolored and eroded or oxidized portions on the surface 103A, since copper may readily react with moisture, oxygen and other traces of gases that may typically be encountered during the removal of the excess material and subsequent substrate handling processes. Consequently, prior to the formation of an insulating capping layer, which may also act as an etch stop layer for the formation of any further metallization layers on top of the substrate 101, the surface 103A is typically cleaned in order to enhance conductivity and the adhesion characteristics and thus the electromigration behavior of the region 103. To this end, frequently a plasma-based treatment may be performed to efficiently remove oxidized, discolored and eroded portions on the surface 103A while at the same time substantially avoiding a re-formation of these portions. For example, a plasma ambient 104 may be established on the basis of ammonia (NH3) and nitrogen (N2), wherein the plasma ignition is typically performed on the basis of radio frequency (RF), the power density of which may significantly determine, in combination with the gas flows of ammonia and nitrogen, the effectiveness of the plasma treatment 104. After the plasma treatment 104, the ambient may be changed by applying appropriate precursor materials so that an appropriate deposition atmosphere may be established in situ, thereby avoiding undue discoloration and oxidation on the exposed surface 103A. After a corresponding stabilization step for introducing the precursor gases, such as 3MS (tri-methylsilane) and ammonia for forming a silicon carbide nitride layer, an appropriate RF power may be supplied to establish a corresponding plasma, thereby initiating the deposition process.
FIG. 1b schematically shows the semiconductor device 100 during this deposition step, wherein, on the basis of the plasma 105 containing 3MS and NH3, a capping layer 106 comprised of nitrogen enriched silicon carbide (SiCN) is formed above the substrate 101 and on the exposed surface 103A, thereby creating a corresponding interface, which is, for convenience, also referred to as 103A. Thereafter, further processing may be continued by forming further metallization layers, wherein the capping layer 106 may act as an etch stop layer for patterning a corresponding dielectric layer stack for forming corresponding vias and trenches.
In other approaches, a silicon nitride layer or a combination of silicon nitride, silicon carbide and nitrogen-enriched silicon carbide may be formed in order to suitably adjust the characteristics. Although good results with respect to the diffusion blocking characteristics may be achieved, significant damage of the exposed copper surface 103A may be observed that is caused by the preceding plasma treatment 104. Moreover, in sophisticated applications, the moderately high permittivity of the capping layer 106 may still result in a significant performance loss. In some approaches, it has been proposed to treat the exposed surface 103A in a plasma with silane and nitrogen to form a copper silicide that may even comprise a significant amount of nitrogen. A corresponding silicide, if sufficiently stable and thick, may replace the capping layer 106, thereby, however, possibly negatively affecting the conductivity of the metal region 103. In other approaches, the plasma treatment is omitted and other pretreatments or deposition regimes for the above-specified dielectric materials may be used, wherein, however, the remaining high permittivity also renders this solution less attractive, while the interface between the copper and the dielectric is still critical in view of electromigration. In still other conventional techniques, the surface 103A is treated to form a silicon and/or nitrogen-containing alloy, allowing omitting the formation of the capping layer 106. In this approach, however, the growth of the alloy is extremely critical and thus process control and device performance may suffer from reduced reliability. Similar difficulties arise from the provision of a copper tungsten/phosphorous alloy, which additionally has to be formed by a wet chemical process, thereby contributing even more to process complexity.
In view of the situation described above, there exists a need for an enhanced technique that enables the formation of copper-based metallization layers including an efficient capping layer while avoiding or at least reducing the effects of one or more of the problems identified above.